This invention pertains to microlithography apparatus and methods, particularly for use in manufacturing semiconductor devices, liquid-crystal display devices, and the like. More specifically, the invention pertains to design and manufacture methods as applied to scanning-exposure microlithography equipment.
Recently, as the feature sizes of semiconductor devices (e.g., memories, processors, custom integrated circuits, etc., as well as displays such in TUFT displays, etc.) have become progressively smaller, the devices themselves have generally increased in size. Generally, such devices are manufactured by processes that include at least one microlithography step.
In xe2x80x9cprojectionxe2x80x9d microlithograpy, a circuit or other feature pattern as defined on a reticle (mask) is projected, using a projection lens, onto the surface of a substrate such as a semiconductor wafer. Microlithography apparatus that perform such multiple exposures are termed xe2x80x9csteppersxe2x80x9d because, after each exposure at a particular site (xe2x80x9cdiexe2x80x9d) on the wafer, the apparatus xe2x80x9cstepsxe2x80x9d to the adjacent die on the wafer (by moving the wafer relative to the reticle) for the subsequent exposure. Usually, the entire reticle pattern is formed on each die on the substrate surface. With xe2x80x9cstep-and-repeatxe2x80x9d steppers, the entire reticle pattern is exposed at the same instant at each die; with xe2x80x9cstep-and-scanxe2x80x9d steppers, the reticle pattern is scanned to expose each die.
In steppers, the projection lens is usually xe2x80x9creducing,xe2x80x9d by which is meant that the image of the reticle pattern formed on the surface of the wafer is smaller (usually by some integer factor such as four or five) than the actual reticle pattern. The projection lens can have reflective elements, a combination of reflective and refractive elements, or all refractive elements.
In many prior-art step-and-repeat steppers, the projection lens has a circular field. The size of each die thus formed on the water surface is limited by the field diameter of the projection lens. As a result, every time a change is required in the size and/or degree of feature resolution of the dies to be formed on the wafer, a new stepper is required. For example, an increase in die size with an accompanying decrease in feature size requires a stepper equipped with a projection lens having a larger projection field (field diameter) and improved resolution (greater numerical aperture).
At least with refractive-type projection lenses, an increase in field size and numerical aperture usually requires an increase in the number and diameter of the optical elements (lens elements) comprising the projection system. This causes much difficulty in the mass production of the projection lenses, especially such lenses that are operable with ultraviolet light sources. For example, projection lenses operable with excimer laser light sources such as 248-nm or 193-nm sources, with a numerical aperture (N.A.) on the substrate side of approximately 0.6 and a projection field diameter of approximately 30 mm, typically comprise at least twenty lens elements. The lens elements can include quartz lenses with diameters of about 130 to 240 mm and fluorite lenses with diameters of about 130 to 170 mm. Such elements are extremely expensive. Moreover, the mass production of large-diameter quartz and fluorite lenses is much more difficult than the manufacture of similarly sized glass lens elements. Thus, the need to design and provide a new projection lens every time there is an incremental change in device size, density, or feature size poses both prohibitive expense and difficulty for both purchasers and manufacturers of steppers.
Step-and-scan steppers as briefly described above recently have been increasingly favored because they are more flexible in accommodating changes in device size, density, or feature size without having to change the projection lens. The principle of step-and-scan systems is discussed in, e.g., J. Vac. Sci. Technol. 17:1147-1155, September/October 1980, in which a reducing projection lens is used with a ring-field (arc-shaped) slit. Step-and-scan can also be employed with a linear slit (part of a rectangular field) as described in, e.g., SPIE, vol. 922 (Optical/Laser Microlithography), pp. 256-268 (1988). A step-and-scan projection exposure device is also disclosed in Japan Kxc3x4kai Patent Publication No. HEI 4-277612, wherein the effective projection field is restricted to a linear slit extending along the diameter inside a circular field.
In the foregoing types of step-and-scan apparatus employing a reducing projection lens, the reticle (mounted on a xe2x80x9creticle stagexe2x80x9d) and wafer (mounted on a xe2x80x9cwafer stagexe2x80x9d) face each other on opposing axial ends of the projection lens. The reticle and wafer must move synchronously at relative velocities that differ from each other by the projection reduction-magnification factor (e.g., ⅕ or xc2xc). Such coordinated movement of the stages must be extremely smooth and accurate at least during scanning and exposure.
Hence, in step-and-scan steppers (as in step-and-repeat steppers), the positioning accuracy and the stepping precision of the wafer and reticle stages are critically important for achieving xe2x80x9ctransfer precisionxe2x80x9d (i.e., faithful reproduction of the reticle pattern on each exposure area with good positional registration and feature resolution). In step-and-scan steppers, unlike step-and-repeat steppers, it is critical that the wafer and reticle stages synchronously move with extreme precision during scanning. Otherwise, transfer precision is unacceptably compromised, resulting in deterioration of image quality from, for example, line-width errors, image distortion, registration errors, and magnification errors.
Certain prior-art step-and-scan steppers as disclosed in, e.g., SPIE, vol. 1088 (Optical/Laser Microlithography), pp. 424-433 (1989) achieve smooth synchronous velocity control of the wafer and reticle stages by driving them with linear motors while using laser interferometers to measure the stage positions. Such control has to be achieved in an environment in which stresses and strains encountered by the stages and their drive mechanisms are always changing.
As a result, each of the mechanisms used for supporting and moving the substrate and reticle stages, as well as the column structures on which the stage mechanisms and projection optical system are mounted, must have an optimal structural design. Representative methods for performing structural analysis simulations of individual assemblies such as the reticle and substrate stages are discussed in, e.g., xe2x80x9cDevelopment of a High-Speed, High-Precision Positioning Stage,xe2x80x9d Proceedings of the 69th Regular Conference of the Japan Society of Mechanical Engineering, vol. C, pp. 11-13 (Apr. 1-3, 1992). Such methods enable one to evaluate the hypothetical properties of a proposed mechanism, such as for a stage, in isolation from other structures.
Unfortunately, a step-and-scan apparatus does not necessarily exhibit a desired transfer precision, even if the various mechanical systems for moving the stages have been optimized. This is because transfer precision is affected not only by the characteristics of the various mechanical assemblies (such as the stages), but also by other factors such as the characteristics of the various control components (e.g., drive motors and laser interferometers) that move and control motion of the stages, characteristics of columns and other supporting structures, air quality and flow inside the chamber in which these subassemblies are contained, the degree to which floor vibrations are isolated from the apparatus, and other factors. Consequently, attempts at optimization of specific assemblies and mechanisms by isolated structural-analysis simulations for each specific mechanism (e.g., vibration-mode optimization), as in the prior art, have been unsatisfactory for accurately estimating the overall transfer precision of a microlithography exposure apparatus from those simulation results.
According to prior-art design and manufacturing methods for steppers, the entire apparatus is assembled from individually optimized mechanical systems and control systems. Unfortunately, it is not possible using conventional methods to accurately determine transfer precision of an entire stepper apparatus from optimization studies performed on individual constituent assemblies. Thus, one must wait either for simulation studies performed after assembly of the entire stepper apparatus is completed or for actual test exposures using the apparatus in order to determine the overall transfer precision. Such methods typically require the construction of multiple prototypes of the apparatus and repeated testing thereof in order to finally provide a stepper apparatus that meets specifications. This requires much time in the development of a new stepper.
In addition, because design problems are inevitably discovered after completing mechanical construction of a new stepper apparatus, additional design, assembly, and testing of the apparatus to rectify deficient components results in even more time required to reach a stage at which the new apparatus can be mass produced. Furthermore, nano-order measurement accuracy is required in order to measure the performance, especially improved performance, of newly revised components or other improvements. Such extreme measurement accuracy demands large-scale facilities for obtaining reliable test data. These factors undesirably increase production costs and delays in the delivery of new microlithography exposure apparatus to customers.
Based on the foregoing, an object of the present invention is to provide design and manufacturing methods for microlithography exposure apparatus that redress the problems summarized above. Specifically, an object of the present invention is to provide such methods that decrease time and costs to develop new microlithography exposure apparatus. Another object of the present invention is to provide design and manufacturing methods that substantially decrease the risks inherent in developing microlithography exposure apparatus in which new functions have been incorporated, especially new functions for which the manufacturer has had no prior design or manufacturing experience. Yet another object is to provide design and manufacturing methods that minimize the time and labor involved in making corrections and later adjustments to the apparatus when problems are discovered during post-manufacturing evaluation of a new apparatus design.
The foregoing objects are met by methods according to the present invention. Such methods generally comprise five phases. In a first phase, a system-analysis model is generated of the kinetic mechanical system (including reticle and wafer stages as well as supporting columns) of the apparatus. The mechanical-system model is generated according to design and engineering specifications for the mechanical system. Also in the first phase, a system-analysis model of the kinetic control system (including electronics, driving motors, and interferometric sensors for controllably moving the stages relative to the column structures) for the mechanical system is generated. The control-system model is generated according to design and engineering specifications for the control system.
In a second phase, an overall system-analysis model is generated encompassing the system-analysis models of the kinetic mechanical system and the kinetic control system. The overall model preferably incorporates any refinements and/or modifications to the mechanical-system model that were made in response to the results of computer-simulated testing of the mechanical-system model.
In a third phase, the transfer precision for the overall system-analysis model is determined, particularly as affected by simulated acceleration and deceleration of the stages, simulated vibrations, and simulated disturbances and perturbations that could affect transfer precision. Such analysis is performed while considering, among other things, values for any relevant parameters such as acceleration and deceleration of the stages, vibrations at any of various locations in the apparatus, and any disturbances or perturbations that could arise from, e.g., air currents, temperature fluctuations, and the like that could degrade transfer precision.
In a fourth phase, if the transfer precision determined in the third phase does not meet a target specification for transfer precision, the design and engineering specifications for either or both the kinetic mechanical system and the kinetic control system are appropriately modified. Afterward, the first three phases are preferably repeated until the transfer precision of the overall model meets the target specification.
In a fifth phase, if the transfer precision determined in either the third or fourth phase meets the target specification, an actual microlithography apparatus is constructed according to the design and engineering specifications for the kinetic mechanical system and the kinetic control system that met the target specification for transfer precision. During the manufacture of the constituent systems, key components and assemblies can be tested and evaluated; if any such tests reveal shortcomings that could degrade performance of the apparatus, the respective model is amended and re-evaluated by re-execution of the first three phases.
According to another aspect of the present invention, a microlithography apparatus is provided for is exposing a circuit pattern defined by a mask onto a substrate. The mask is mounted to a movable first stage and the substrate is mounted to a movable second stage. The apparatus comprises a computer that is operable to perform general controlling of the apparatus and to carry outran exposure operation of the subtrate by moving the first or second stages according to a program entered into the computer by a user. The apparatus also comprises a database memory connected to the computer. The database memory is operable to store a characteristic of an overall system-analysis model that is able to simulate an exposure accuracy or a transfer precision occurring with a motion of the first or second stages and actual disturbances or perturbations encountered under actual operating conditions.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.